The Endocrine System
347
Environmental Factors
Infl uencing Growth
Adequate nutrition and freedom from disease are the primary
environmental factors infl uencing growth. Lack of suffi cient
amounts of amino acids, fatty acids, vitamins, or minerals
interferes with growth. Total protein and nutrients to provide
energy must also be adequate.
The growth-inhibiting effects of malnutrition can be
seen at any time of development but are most profound when
they occur early in life. Thus, maternal malnutrition may cause
growth retardation in the fetus. Because low birth weight is
strongly associated with increased infant mortality, prenatal
malnutrition causes increased numbers of prenatal and early
postnatal deaths. Moreover, irreversible stunting of brain
development may be caused by prenatal malnutrition. During
infancy and childhood, too, malnutrition can interfere with
both intellectual development and total body growth.
Following a temporary period of stunted growth due to
malnutrition or illness, and given proper nutrition and recov-
ery from illness, a child can manifest a remarkable growth
spurt
(catch-up growth)
that brings the child up to the nor-
mal height expected for his or her age. The mechanisms that
account for this accelerated growth are unknown.
Hormonal Infl uences on Growth
The hormones most important to human growth are growth
hormone, insulin-like growth factors I and II, thyroid hor-
mones, insulin, testosterone, and estrogens, all of which exert
widespread effects. In addition to all these hormones, a large
group of peptide
growth factors
exert effects, most of them
acting as paracrine and autocrine agents to stimulate differen-
tiation and/or cell division of certain cell types. The general
term for a chemical that stimulates cell division is a
mitogen.
There are also peptide
growth-inhibiting factors
that
modulate growth by inhibiting cell division in specifi c tissues.
The growth factors and growth-inhibiting factors are usually
produced by multiple cell types rather than by discrete endo-
crine glands.
The various hormones and growth factors do not all
stimulate growth at the same periods of life. For example,
fetal growth is largely independent of growth hormone, the
thyroid hormones, and the sex steroids, all of which stimulate
growth during childhood and adolescence.
Growth Hormone and Insulin-Like
Growth Factors
Growth hormone, secreted by the anterior pituitary gland, has
little or no effect on fetal growth, but is the most important
hormone for postnatal growth. Its major growth-promoting
effect is (indirect, as we will see) stimulation of cell division in
its many target tissues. Thus, growth hormone promotes bone
lengthening by stimulating maturation and cell division of the
chondrocytes in the epiphyseal plates, thereby continuously
widening the plates and providing more cartilaginous material
for bone formation.
Figure 11–26
Anatomy of a long bone during growth.
Epiphysis
Epiphyseal
growth
plate
Marrow
cavity
Shaft
Epiphysis
100
80
60
40
20
4
Total growth (%)
Age (years)
Reproductive organs
Brain
Total body height
81
2
2
0
16
Figure 11–27
Relative growth in brain, total body height (a measure of long-
bone and vertebral growth), and reproductive organs. Note that
brain growth is nearly complete by age 5, whereas maximal height
(maximal bone lengthening) and reproductive-organ size are not
reached until the late teens.
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